Last Saturday, Feb 2, I was in the southern end of the Santa Rita Mts. doing a springs assessment for Sky Island Alliance and I found a manzanita bush in bloom and teaming with life. I thought this very unusual as that morning there had been frost on the ground and for several weeks the temperature had been very cold. I took some pictures which are listed below. Manzanita (Arctostaphylos sp.) produces masses of bell-shaped white to pink flowers with fused corollas that restrict the opening. They are among the first to flower in late winter and are sometimes the only flowering plants in January or February. They dominate the chaparral life zone and are quite common up into the timber zone. I noticed what looked like round black spots on many flowers. These turned out to be small round holes cut in the flowers by nectar robbers. There were also vertical slits. Nectar robbers collect nectar from the base of the flower but don’t pollinate the plant so they are cheaters in any mutualist system between plants and pollinators. Richardson and Bronstein, 2012 have investigated this phenomena. They identified a range of six behaviors from 46 different insect taxa found visiting the flowers. These behaviors are:
- Normal nectar collection
- Normal pollen collection
- Buzz collection – the insect straddles the mouth of the flower and vibrates it’s legs, collecting pollen as it falls out.
- Forced Entry – the insect forces its legs down into the mouth of the unopened flower. Only nectar is collected.
- Hole cutting – a hole or slit is cut near the base of the flower, sometimes more than one
- Secondary robbery – insects visit already cut holes or slits
The authors assumed that the first three of these behaviors are beneficial to the plant and the last three are detrimental. This was an assumption only as there is no information on actual fitness benefit. They determined that Arctostaphylos pungens does not self-pollinate successfully. Also, there are no morphological changes to the flower such as a thickened base to discourage nectar robbery. I could find no evidence of ant mutualism protecting the nectary. It is unknown how such a system of cheating with no known countermeasures is stable for the plant. Yet, manzanita thrives.
Nectar robbery was only done by 2% of the visiting insects, but accounts for an 83% loss of pollen. The main reason for this is secondary robbery. All insects, no matter what their additional strategies, make normal collecting a majority behavior. This is puzzling because a successful cheating strategy that is beneficial to the insect would tend to dominate as a behavior, regardless of it’d effect on the plant. The same species of insects have multiple strategies and perhaps even individual insects have multiple strategies. Mutualism between plants and pollinators happens but with pollination, a specialist strategy vs a generalist one is in the minority. This is just the opposite of herbivory where specialists seem to rule. A major question is how can plant/pollinator adaption take place amidst generalists?
I looked in the USDA PLANTS database at the Arctostaphylos genus. There are 77 identified species in the US. The database has images of many species. I found four species with images of flowers with nectar robber damage. Also, all the images show the same shaped flower, the major morphological difference in the outside of the flower being the color. So this suggests a constancy of flower morphology over the genus. In Richardson and Bronstein, 2012 there is mention of A. pringlei, another Arizona species. The flowers of this plant have a much more variable opening than A. pungens and the authors mention bees robbing flowers with small openings and not flowers with large openings. This suggests a relationship between bee size and flower opening. The round cut openings look a bit like the mouth of the flower. Flower buzzing and maybe even forced entry could also be behaviors related to flower openings too small for the bee. This seemed reasonable to me as there could be some relationship between flower morphology and what is happening. This naive theory of mine was confounded by a correspondence with one of the authors, Dr. Leif Richardson. The only observed nectar robbery took place with bees cutting slits in the plants. No one saw what was cutting the holes. There is another system of association going on between the flowers and thrips (Orothrips kelloggii and two other species) that live in the flowers. This could also involve pollination (García‐Fayos and Goldarazena, 2008). The thrips could be cutting the holes or perhaps a predator of thrips is doing this. It is known that hummingbirds visit the flowers and perhaps prey on trips but they will destroy the flower in the process. So it looks like I might have to spend some time observing this. Funny what a few minutes with a camera can get you into.